1. Field of the Invention
The present invention relates to a sealing device for a reciprocating shaft which seals an axial periphery of the reciprocating shaft, for example, a piston rod of a hydraulic shock absorber.
2. Description of the Conventional Art
As a typical prior art of the sealing device for the reciprocating shaft which is used as a means for sealing the axial periphery of the reciprocating shaft such as the piston rod of the hydraulic shock absorber of a vehicle, for example, there is a structure described in Japanese Patent Application Laid Open No. 2001-173797.
FIG. 3 is a half sectional view illustrating a conventional sealing device for a reciprocating shaft of the same kind as the above-described patent document by cutting along a plane passing through an axial center. In FIG. 3, the sealing device 100 is held between a housing, which is not illustrated in drawings, and a reciprocating shaft. The sealing device 100 has a main lip 101 and a dust lip 102 which are integrally formed with a rubber-like elastic material at an inner peripheral part of a flat washer-shaped metal ring 104, and has an outer peripheral seal part 103 which is integrally formed with a rubber-like elastic material at an outer peripheral part of a reinforcement ring 104.
The main lip 101 is directed on the sealing space A side, and an extension spring 105 for compensating tension force is provided in it. By tightly contacting a seal projection 101a at an inner periphery with a proper interference on an outer peripheral face of a reciprocating shaft 200, the main lip 100 prevents to flow out an operating oil, which is sealed in the sealing space A, to an external space B. By tightly contacting an inner peripheral end part 102a with a proper interference on an outer peripheral face of the reciprocating shaft 200, the dust lip 200 prevents to invade a foreign matter into the sealing space A. Further, a supporting projection 101b is formed on the opposite side of the sealing space A (the dust lip 102 side) for controlling deforming of the main lip 101 by the operation oil or the like in the sealing space A. This supporting projection 101b is formed with respect the seal projection 101a of the main lip 101.
However, in the conventional sealing device for a reciprocating shaft 100, since the main lip 101 for sealing the operating oil serves commonly as a frictional force providing force to the reciprocating shaft 200, the required frictional force may not be provided. Further, the frictional force of the main lip 101 with respect to the outer peripheral face of the reciprocating shaft 200 is largely varied also by the fluctuation of the oil pressure in the sealing space A. So, an oil film intervening between the outer peripheral face of the reciprocating shaft 200 and the seal projection 101a of the main lip 101 becomes thin so as to short in lubrication depending on the oil pressure in the sealing space A. Thus, since the seal projection 101a is abraded in an early stage, frictional coefficient is varied. So, it is hard to keep stable frictional force for a long period of time.